The harmful effect of the ultraviolet part of solar radiation on the skin is generally known. Depending on their particular wavelength, the rays have different effects on the skin as an organ:
The so-called UV-C radiation with a wavelength between 100 and 280 nm is absorbed by the ozone layer in the Earth's atmosphere and accordingly is not found in the solar spectrum. It is therefore of no physiological importance during sunbathing.
The so-called UV-B region is between 290 nm and 320 nm. UV-B rays are essentially responsible for the long-lasting tanning of the skin, but can at the same time cause an erythema, simple sunburn or even burns of greater or lesser severity. Chronic photodamage, photodermatoses and Herpes solaris can also be caused by UV-B radiation.
It has for a long time been incorrectly assumed that long-wave UV-A radiation with a wavelength between 320 nm and 400 nm only has a negligible biological effect and that, correspondingly, the UV-B rays are responsible for most photodamage to the human skin. However, in the meantime, numerous studies have studied that UV-A radiation is much more hazardous than UV-B radiation with regard to the triggering of photodynamic, specifically phototoxic reactions and chronic changes in the skin. The harmful influence of UV-B radiation can also be further intensified by UV-A radiation.
Thus, it has, inter alia, been found that even UV-A radiation suffices under very normal everyday conditions to harm, within a short time, the collagen and elastin fibers which are of essential importance for the structure and strength of the skin. The consequences are chronic photo-induced changes in the skin—the skin “ages” prematurely. The clinical appearance of skin aged by light includes, for example, wrinkles and lines, and also an irregular, furrowed relief. In addition, the parts affected by photo-induced skin aging have irregular pigmentation. The formation of brown spots, keratoses and even carcinomas or malignant melanomas is also possible. Skin aged prematurely by everyday UV exposure is, moreover, characterized by lower activity of the Langerhans cells and slight, chronic inflammation.
Approximately 90% of the ultraviolet radiation which reaches the Earth consists of UV-A rays. While UV-B radiation varies widely depending on numerous factors (e.g. time of year and time of day or degree of latitude), UV-A radiation remains relatively constant day to day irrespective of the time of year and time of day or geographical factors. At the same time, the majority of UV-A radiation penetrates into the living epidermis, while approximately 70% of UV-B rays are retained by the horny layer.
The relatively recent findings concerning the effect of UV-A rays on the skin have led to increased attention now being devoted to protective measures for this ray range. In practice, no sunscreen product is complete any more without an effective UV-A filter effect, and pure UV-B filter preparations are rare.
When applying a sunscreen to the skin, the ultraviolet rays can be weakened through two effects: firstly, by reflection and scattering of the rays at the surface of pulverulent solids (physical light-protective) and, secondly, by absorption on chemical substances (chemical light-protective). Depending on which wavelength region is absorbed, a distinction is made between UV-B filters (absorption range 280 to 320 nm), UV-A filters (absorption range 320 to 400 nm) and broadband filters (absorption range 290 to about 380 nm).
To protect against UV-B radiation, numerous compounds are known, the absorption maximum of which should be around 308 nm as far as possible since this is the highest erythema effectiveness of solar radiation. Typical UV-B filters are, for example, derivatives of 3-benzylidenecamphor, of 4-aminobenzoic acid, of cinnamic acid, of salicylic acid, of benzophenone, and also of 2-phenylbenzimidazole.
Some compounds are also known for protecting against UV-A radiation, such as, in particular, dibenzoylmethane derivatives. However, dibenzoylmethane derivatives are generally not photostable, as a result of which cosmetic or dermatological preparations with a content of this substance should also comprise certain UV stabilizers. Further known UV-A filter substances are certain water-soluble, sulfonated UV filter substances, such as, for example, phenylene-1,4-bis(2-benzimidazyl)-3,3′-5,5′-tetrasulfonic acid and its salts.
Besides the pure UV-A or UV-B filters, there are substances which cover both regions. This group of broadband filters includes, for example, asymmetrically substituted s-triazine compounds, such as, for example, 2,4-bis{[4-(2-ethylhexyloxy)-2-hydroxy]phenyl}-6-(4-methoxyphenyl)-1,3,5-triazine (INCI: BisEthylhexyloxyphenol Methoxyphenyl Triazine), certain benzophenones, such as, for example, 2-hydroxy-4-methoxybenzophenone (INCI: Benzophenone 3) or 2,2′-methylenebis(6-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol) (INCI: Methylene Bis-Benzotriazolyl Tetramethylenebutylphenol).
In general, the light absorption behavior of light-protective filter substances is very well known and documented, especially as there are positive lists for the use of such substances in most industrialized countries, which impose very strict standards on the documentation. Since, in order to characterize a filter substance, not only is the position of the absorption maximum important, but primarily the absorption range, absorption spectra are recorded for each substance. However, the absorbance values can at best be a guide for the concentration of the substances in the finished formulations since interactions with ingredients of the skin or of the surface of the skin itself may give rise to imponderables. In addition, it is usually difficult to estimate beforehand how uniformly and thickly the filter substance is distributed in and on the horny layer of the skin.
To test the UV-A protection performance, use is usually made of the IPD method (IPD≡immediate pigment darkening). Similarly to the determination of the sun protection factor, this method gives a value which indicates how much longer the skin protected with the light-protective composition can be irradiated with UV-A radiation until the pigmentation which occurs is the same as for the unprotected skin.
The use concentration of known light-protective filter substances present in the form of a solid, which exhibit a high filter effect in the UV-A region is, however, often limited—especially in combination with other substances to be dissolved. This therefore gives rise to certain technical difficulties relating to formulation in achieving relatively high sun protection factors or UV-A protection performance.